1. Field of the Invention
This invention relates to an apparatus for inspecting the appearance of cylindrical objects and more particularly to an apparatus for automatically inspecting abnormality in external surface, dirt on the surface, and poor shape of cylindrical objects by using image processing.
2. Description of the Prior Art
In industrial mass production, automation of producing and inspecting systems has been pushed ahead to rationalize the production process. The inspection of the appearance of products, which is one of the matters to be rationalized, is mainly performed through visual observation. However, in recent years with the spread of computers, it has developed that images which are picked up by a television camera (hereinafter merely called as "camera") are processed by a computer. The technic in which the picked up image is processed by the computer has been developed and adapted to automatically inspect the appearance of the products.
In the apparatus for inspecting the appearance with image processing, the images picked up by the camera are converted into analog signals. Then, the signals are converted into digital signals so as to be discriminated by the computer.
As a method of discriminating or inspecting the appearance with a computer, it has been generally used that an image of a standard product or an acceptable product (hereinafter merely called as "a standard image"), which is stored in the computer in advance, is compared with an image of an inspected object (hereinafter merely referred as "an inspected image"). Then, if there is a point or portion of the inspected image that does not correspond with the standard image, the inspected object relating to the inspected image is judged to be a defective. On the other hand, when the standard image and the inspected image coincide with each other, the inspected object relating to the inspected image is judged to be an acceptable product. In the discriminating method using such a standard image, even though characters or symbols are printed on the external surface of the inspected object, the inspecting operation can be carried out in consideration of the existence of the characters, symbols or the like.
Besides the above method, there is another method in which luminance signals of the image, which is picked up by the camera, are digitized and the luminance is integrated in a prescribed range to judge the inspected object as a defective product when the integrated luminance falls below (or is less than) a predetermined threshold.
As described above, in the method of comparing the standard image with the inspected image, an entrapping start point and a finishing point must always coincide with those of the standard image, and the inspected object is limited to be a polygon with symmetrical shape.
When the appearance of the cylindrical object is inspected based on the image picked up by the camera, an overall image of the object including external periphery should be entrapped. As described above, in the method of inspecting the appearance of the cylindrical object, the inspected object is rotated while the camera being fixed or the camera is rotated about the inspected object to pick up the appearance of the inspected object. The method in which the inspected object itself is rotated while the camera is fixed is preferable since in the other method, mounting and rotating mechanisms for the camera become rather complicated.
Further, in the method of rotating the inspected object, the rotating operation should change in accordance with the diameter of the inspected object. That is, when the diameter is small, the inspected object is preferably rotated with an axis of the horizontally laid inspected object. Otherwise, the structure for maintaining and supporting the inspected object becomes complicated.
However, in the method of inspecting the appearance of the cylindrical object by entrapping the overall image of the object, it is difficult to compare the inspected image with the standard image since the start point of entrapping the inspected object does not always coincide with that of the standard image.
That is, in mass production of the cylindrical object, the appearance of the inspected objects must be inspected while the object is automatically transported by conveyors or the like. Therefore, it is difficult to fix the start point of the image of the inspected object at the inspection of the appearance thereof due to the mechanism of the transporter. To eliminate the problem, not only will the transporting mechanism become complicated but also the speed of the inspection will be decreased.
As a result, it is proposed that the luminance of the inspected object is summed up to inspect the appearance of the cylindrical object, which requires the appearance inspection consider of characters or symbols on the outer periphery of the inspected object.